This invention relates to bit rate reduction and expansion and, more particularly, to methods and apparatus for processing nonuniformly encoded digital words.
Many digital compressors have been developed in the prior art for use with uniformly encoded analog signals. The digital compressor disclosed in U.S. Pat. No. 3,789,392 to G. Candiani, dated Jan. 29, 1974, is representative of these digital compressors of the prior art. In these compressors a uniformly encoded digital word is reduced in size prior to tranmission by translating the number of digital zeros which follow the initial sign bit into a 3-bit code word. The 3-bit code word is then followed with a predetermined number of bits which follow the initial digital zeros. In the case of large magnitude digital words many of the less significant digital bits are simply discarded.
The digital format developed in the Candiani patent is similar to the format developed by encoders in the D2 and D3 channel banks of the Bell System, utilized to convert analog signals into a nonuniformly encoded digital bit stream. These channel banks in their process of encoding analog signals nonuniformly quantize these signals in order to improve the signal-to-noise ratio of the low level signals at the expense of a coarse quantization of the high amplitude analog signals. The nonuniform encoding utilized by the channel banks in the Bell System is known to those skilled in the art as .mu.-law encoding. In the .mu. = 255 nonuniform law utilized in the D2 and D3 channel banks of the Bell System, each analog sample is converted into an 8-bit binary code. The initial bit of the digital word represents the sign or polarity of the analog sample while the coarse amplitude of the analog sample is represented by the three bits following the sign bit, each value of this 3-bit word representing one of eight amplitude ranges known as segments or chords. The length of these segments double in magnitude as the signal becomes larger in magnitude. Finally, the remaining four digital bits represent one of 16 equal-length intervals present in each one of the segments or chords.
Still another form of digital bit rate reduction for use with uniformly encoded analog samples is known to those skilled in the art as nearly instantaneous digital companding or as block companding. This technique of nearly instantaneous companding is described in the article by M. G. Croll, M. E. B. Moffat and D. W. Osborne entitled "`Nearly Instantaneous` Digital Compandor for Transmitting Six Sound-Programme Signals in a 2.048 Mbit/s Multiplex," Electronics Letters, July 12, 1973, Vol. 9, No. 14, pages 298-300, and in the article entitled "Progress in PCM and Delta Modulation: Block-Companded Coding of Speech Signals," by A. Croisier, IBM France Center d'Etudes et Recherches, 06610 - La Gaude, France, from a presentation at the 1974 International Zurich Seminar on Digital Communications, March 12-15, 1974.
In the technique of nearly instantaneous companding the uniformly encoded samples are processed in blocks of a predetermined number of samples. The predetermined number of encoded samples in each block is stored in a digital delay while the largest sample for the block is determined. The value of this largest sample is then utilized to process the digital words stored in the delay. The position of the most significant "1" in the largest sample is represented by a scale factor word and this word is then multiplexed along with the processed digital words in transmitting information to a receiving location.
In the nearly instantaneous compandor described in the article by M. G. Croll et al., the peak value in a block of digital words is determined by searching for the most significant digital 1 in the block of digital words. The position for this most significant digital 1 is then utilized by the processor in determining the most significant bit of a fixed length word to be transmitted for each of the digital words in the block. The block processing described in the A. Croisier article carries out a similar technique of bit rate reduction with an additional feature for determining the maximum of the differences between successive samples if differential pulse code modulation coding is considered. In both forms of prior art block processors, uniformly encoded analog samples are present at the input of the apparatus.